(1) Field of the Invention:
This invention relates to a polypropylene-base resin composition having excellent low-temperature impact resistance, high molding fluidity, superb paintability and high stiffness.
(2) Description of the Prior Art:
Although polypropylene resins have heretofore been used widely in various fields because of their excellent physical properties such as low specific gravity, high stiffness, and superb chemical and heat resistance, they are accompanied by a drawback in that they have poor impact resistance at low temperatures. With a view toward overcoming this drawback, it has been practiced to use propylene-ethylene copolymers as polypropylene resins or to blend a rubber-like substance, such as an ethylene-propylene copolymer rubber, or polyethylene with polypropylene resins. In addition, it is also commonly practiced to improve their stiffness, heat resistance, dimensional stability, paintability and the like by incorporating a variety of fillers in polypropylene resins.
Given these conditions, it is, however, required for polypropylene resins to fulfill mutually contradictory properties such as high molding fluidity and as moldings, high stiffness, high heat resistance, easy paintability and high impact resistance. Moreover, each of the above properties is required at a high level.
A variety of improvements have been proposed for the above requirements to date. Most of these proposals are, however, insufficient to meet and balance the high degrees of physical properties required. Therefore, it may be required in many instances to add a rubber component, fillers and so on at higher concentrations to polypropylene resins or to apply a special treatment to them, resulting in increased costs. Although subject to the shapes of moldings to be molded, use of such resin compositions may result in moldings having poor external appearance, especially, due to noticeable weld lines, thereby reducing their commercial values. It is therefore strongly desired to make improvements to these drawbacks.
The present inventors have already proposed a process for preparing a polypropylene-base resin composition, which comprises heating at a temperature of 170.degree. C.-280.degree. C. a mixture containing a crystalline ethylene-propylene block copolymer of a specific composition, an ethylene-propylene copolymer rubber of specific composition and viscosity, an inorganic filler of a specific particle size and an organic peroxide, respectively, in specified amounts (see, WO No. 85/00824).
The ethylene-propylene copolymer rubber, which is useful in the practice of the above process, is limited to that having a propylene content of 40-65 wt. % and a Mooney viscosity ML.sub.1+4 of 15-80 at 100.degree. C.
Although the above-described process is an excellent process, the resultant resin composition gives moldings having poor surface gloss at unpainted parts thereof when it was molded. A still further improvement is therefore desired.